The present invention relates to a method and apparatus for making compensation for a DC offset voltage generated in an amplifying circuit in an electromagnetic flowmeter without being affected by AC noise generated between electrodes of the electromagnetic flowmeter.
In magnetic induction flow measurement in which a flow rate of a fluid is measured by means of a DC magnetic field having a periodically reversed polarity, a method of making compensation for a DC offset voltage generated in an amplifying circuit in an electromagnetic flowmeter is known as disclosed in JP-A-No. 57-146113, the method comprising the steps of: sampling and storing an effective signal voltage in each of sampling periods where the magnetic field takes a positive polarity and a negative polarity respectively; forming a difference between the sampling values; sampling and storing the signal voltage in a compensation period following each sampling period to thereby produce a compensation voltage to be superimposed onto the signal voltage; making compensation on the signal voltage by using the compensation voltage to thereby make the signal voltage zero and holding the compensation voltage until a succeeding compensation period.
However, an actual trouble voltage can not be regarded as a simple DC voltage and has a large variation which can not be disregarded even in half period of excitation.
Specifically, problems are as follows.
(1) Electrochemical noises having a wide frequency range are generated across the electrodes and the magnitude and frequency components of the noises vary depending on the kind and amount of ions in a fluid, and materials and surface conditions of the electrodes. PA1 (2) Pulse-like noises having widths within a range of from several msec to several tens msec may be generated in a case of a slurry fluid. PA1 (3) Random noises which rapidly increase when the flow rate in the measurement tube rises 3.about.4 m/sec or more, and such random noises (hereinafter referred to as "a high velocity fluid noise" may be significant trouble noises. FIG. 6 shows an example of actually measured data of the relationship between the frequency and magnitude of such a high velocity fluid noise. As seen in FIG. 6, the high velocity fluid noise is a gathering of a number of noises within a wide frequency range and has particularly large magnitudes in the vicinity of the frequency of the excitation current, that is, at about one.about.several tens Hz. PA1 (4) On the other hand, there is a tendency to make the excitation current small to reduce the power consumption in the electromagnetic flowmeter, and the signal voltage is drastically reduced to about 10 .mu.V per 1 m/sec of flow rate.
In the case of such a very small voltage signal being used, the foregoing AC noises superimposed on a flow signal can not be disregarded.
In the conventional method in which a signal voltage is compensated to be zero in a compensation period, there have been problems in that large variations are caused in the amplified output when such an AC noise is superimposed on a flow signal, and particularly in that very large variations are caused when a pulse-like noise is superimposed on a flow signal in a compensation period.
Further, in the prior art, a differential amplifier in the input stage is directly coupled with electrodes. Accordingly, it is necessary to reduce the amplification degree of the differential amplifier to about 1.about.10 so that the differential amplifier cannot be saturated by a trouble DC voltage. Accordingly, there has been a problem in that the amplification degree of the input stage is not sufficient in a low power consumption electromagnetic flowmeter in which very small signals not larger than 10 .mu.V per 1 m/sec of flow rate are used.